Disk structure.45

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Disk Structure

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Recap

In the last class, you have learnt:

• Working set model

• Page fault frequency

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Objective

On Completion of this period, you would be able

to know :

• Requirements of secondary memory

• Types of secondary memory

• Disk structure

• Head types

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Secondary Storage Management

Ideal requirement – Programs and data must reside in main memory permanently for process execution but,

Constraints:

• Main memory is a volatile storage

• Too small

• Expensive

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Requirements of Secondary Memory

• Should hold large amounts of data

• Should be nonvolatile

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Types of Secondary Memory

• Magnetic tapes

• Magnetic disks

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Types of secondary memory

• Magnetic Tapes

– Early secondary storage device – Slow, cannot provide random access– Currently used for backup of infrequently used

information

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Types of Secondary Memory

• Magnetic Disks

– Fast and provides random access

– Provides bulk of secondary storage

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Disk Structure

Fig 1 moving head disk mechanism

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Physical Structure

• Flat circular Shape

• Two surfaces are covered with magnetic material

• Information is recorded on both surfaces

• The disk surface is logically divided into tracks

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Operating System View of Disk

• One dimensional array of disk blocks

• Each block is a sector

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Mapping of Disk address

Disk address

Cylinder i

Surface j

Sector k

Block number = k + s ( j + i * t )

Operating System view of Disk

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Working Principle

• The drive motor spins, disk at high speed

• The read write head is positioned just above the

surface of the disk

• Information is stored magnetically on the track

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Head Types

• Fixed-head disk

• Moving-head disk

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Fixed-head Disk

– Separate head for each track

– High speed access

– Large number of Heads

– Very expensive

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Moving-head disk

– Only one head

– Moves in and out to access different tracks

– Less expensive

– Additional hardware required to move the head

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Increasing Storage Capacity

• Improving recording density

– Increasing number of tracks per inch

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Increasing Storage Capacity

• Using two sides of the platter for storing information

– Additional read write head required

– Storage capacity is doubled

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Increasing storage capacity

• Stacking several disks

– Additional read/write heads are required for each disk

– Only one motor drive is sufficient

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Disk Characteristics

• Read, modify and write it back in the same place

• Access any block directly

• Sequential or random access is allowed

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Summary

In this class, you have learnt

•Types of secondary memory

•Disk structure

•Head types

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Frequently Asked Questions

1. Explain Disk structure

2. Write a note on the Secondary storage devices

3. Explain different types of R/W heads

4. Explain the head disk characteristics

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Quiz

• Moving Head is more expensive than fixed head mechanism [T/F]

• Storage capacity can be increased by

•False

Increasing density Using two surfaces of the platter Stacking several disks All the above

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Quiz

• Magnetic tapes provide direct access [T/F]

• The disk has a Flat circular shape [T/F]

False

True

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